Floating point numbers are stored as approximations in computers due to how binary systems handle decimals so they may not store the exact decimal value. For example, a number like 5.15 might be stored as 5.1499999999999. This can sometimes lead to some unexpected results which may be contributing to this discrepancy. The best approach in this situation is to store the result of the expression in a separate variable and then apply the Trunc() function to the stored result as seen in your second attempt above.

u5 := ((u1 - u2) / 2 - u3) / u4;
result2 := Trunc(u5);